The Wireshark developer community, http://www.wireshark.org

Wireshark

Wireshark (previously known as Ethereal) is a free network protocol analyzer. Statisics, burst size, output buffer size, losses for multicast data, response time, next and previous marked frames, track and share names, encoded files, dissectors, Ethernet, Token-Ring, serial, etc. It is useful for analysis, troubleshooting, development, and education.